Electric Shocks

How an electric shock occurs…

Current travels through conductors within a closed circuit. The conductors could be made of copper such as in a cable, other metals, water or in the worst case the conductor can become the human body.

An electric shock occurs when a person becomes part of a circuit, with current entering the body at one point and leaving it at another.

The two methods of receiving an electric shock are…

Coming into contact with a normally live part (i.e. touching the live terminals within a consumer unit) or by touching a non-live part that has become live due to a fault in an electrical installation such as the case of a washing machine that has become live due to an earth fault

How serious the shock you receive depends on your physical condition, and general health.

The amount of current present – it’s the current that causes the problem and not the voltage.

And the path the current flows through your body and the length of time exposed to the current present.

So, remember, Voltage hurts but Current can kill

Non-lethal electric shocks occur when…

More than 3 milliamps (0.003A): You’ll receive a painful shock
More than 10 milliamps (0.01A): You’d feel muscle contraction
More than 20 milliamps (0.02A): This would really hurt and is considered severe electric shock
More than 30 milliamps (0.03A): You’d receive lung paralysis – usually temporary

Lethal electric shocks are…

More than 50 milliamps (which is only 0.05A): Possible ventricular fibrillation ‘heart flutter’ which is usually fatal
100 milliamps (0.1A) to 4 amps: Certain ventricular fibrillation and definitely fatal
Over 4 amps: Heart paralysis / severe burns and very fatal – electric chair!

If you see someone receiving an electric shock
Isolate the person from the electrical source. Turn off the electricity if possible, or move the person using a non-conducting material such as wood.
Never touch the person receiving an electric shock or you too could receive one.

Other first aid will quite likely be needed

The 17th Edition Has Still Got Legs

The 17th Edition has still got legs…

Although the 17th Edition of BS 7671 is due to be replaced by the 18th Edition in January 2019, the 17th Edition is relevant today and will still be as relevant long after that date.

The main reason for this is…

Any installation designed before January 2019 would only need to comply with the 17th Edition. Meaning, you could be installing to the 17th Edition well into the 2020s and even beyond.

And for anyone already with the 17th Edition including Amendment 3…

There’s usually an overlap period, as electricians are not expected to drop everything and head for the exam centres on the 1st of January 2019.
Plus, going off experience, the City & Guilds 2382-19? exam won’t be available till well after that date.

Qualified Supervisors, you’d probably be expected to update sometime during 2019 and within 2020 at the latest. The same goes for small business owners and JIB electricians – agency sparks, subcontractors etc. ‘Don’t forget to update your JIB card’.

It’s easier…

What I mean is, I’ve recently been going through the draft copy of BS7671 18th Edition and apart from it having around two hundred and fifty more pages (50% bigger). There’s plenty of new regulations, including a whole new part, ‘Part 8 – Energy Efficiency’.

“I won’t bore you with all the details of the new book for now, I’ll save that for a future post.”

This means that they are not just going to have to rewrite the whole exam for the 18th Edition, mainly to include the addition of Part 8, but the whole exam will have to be reconfigured.

This could mean a re-jig of the current exam with fewer questions on say Parts 4 and 5 or even the possibility of additional questions, raising the number of exam questions to above the current sixty.

The 18th Edition exam will be based on a bigger book with more pages to reference than the 17th Edition and in all likelihood, there will be more questions too.

However, there’s usually an update exam which will be opportunity for people who already have the 17th Edition up to Amendment 3 to take a shorter exam that’s just based on the changes introduced to the 18th Edition of BS 7671

In my opinion, passing the 17th and then the passing the 18th Edition update exam will be easier than completing the whole of the 18th Edition exam in one go.
So, the 17th Edition will be relevant well after January 2019, and passing it before then will make it easier to get your 18th Edition when the time comes.

Competent Person’s Schemes

In order to be able to sign off your own work under Part P you would need to be a member of a domestic installers competent person’s scheme ie with NAPIT, Elecsa, NICEIC etc.

To join a competent persons scheme you would need to demonstrate to them that you have sufficient knowledge and experience to complete your work safely and in compliance with any applicable regulations. The basic qualifications that could be used as evidence of knowledge are the 17th Edition Wiring Regulations C&G 2382, the Building Regulations C&G 2393 and an Inspection and Testing qualification such as the C&G 2392 ‘Initial Verification and Certification’.

These combined with evidence of experience such as references as well as work you have completed that they can examine could be enough to secure your place on a competent person’s scheme. Although installers with little experience may be required to complete a more formal – apprentice type course and qualification such as the old C&G 2365 up to at least level 2.

Our online courses at partptrained.co.uk will provide the qualifications for the Building Regulations C&G 2393 and the Inspection and Testing C&G 2392 and our online workshop contains the domestic electrical installation theory training needed to be a good competent domestic installer.

We also provide the online training and the C&G 2382 exam for the 17th Edition Wiring Regulations at our online training website: www.the-regs.co.uk

If you ring round the competent person’s scheme providers – the NICEIC may require more experience, but Elecsa and NAPIT are usually very helpful.

Approved Document P – Expained.

What is an Approved Document?

Approved documents are a series of documents that have been approved by the government and give practical ‘non-statutory’ guidance on how to meet the requirements of the Building Regulations which are a statutory (legal requirement).

If you follow the guidance within an approved document there is a presumption that you will comply with the requirements of the building regulations that it applies to.

“In plain language: If you comply with what is written in Approved Document P then you should comply with Building Regulation Part P.”

Part P of the Building Regulations came into effect in April 2013 and is for use in England.
Scotland come under the Building (Scotland) Act 2003 and Wales at this time are working to Approved Document P – 2006 updated to 2010.

There’s more information regarding Part P over at our new website www.partPtrained.co.uk 

‘Part P Trained’ is online training and seven day support for the Part P and the City & Guilds 2393 Building Regulations course and includes our very own domestic electrical installer’s online Workshops and offline training and is available today.


Installing a Shower – Electrically !

Wiring a Shower.

Perhaps one of the easiest circuits to install is the one that feeds a shower. Although it’s just a simple radial circuit there are a few pit holes you’ll need to avoid. We are going to assume that we are dealing with a domestic installation and as such Volt Drop will not be an issue, unless the shower is on the 3rd floor over on the east wing.

Size of cable required?

More than likely it will be a 10kW shower these days and it will use 10000/230 = 44A. So, we’ll need a cable that can carry 44A or more.

Looking at Table 4D5 BS 7671 we see that if we have the cable clipped direct (ref method C) we could use a 6mm sq twin and earth as it will carry up to 47A

However, it’s not a simple as that and it’s also highly unlikely that it’s going to be clipped directly throughout the building and we must allow for that.

Taking the scenic route and avoiding too much thermal insulation we’ll settle for reference method 100 and this will give us 45A current carrying capacity for a 10mm sq twin and earth.

Size of circuit breaker?

Have a look at Table 41.3 in BS761, not for the maximum Zs values for these circuits but for the size of circuit breakers commonly available.

You’ll notice that there is a 40A circuit breaker but, that would be too small as every time the shower was switched on it would trip. The next one shown is a 50A circuit breaker, but again this is no good as it is higher than the current carrying capacity of the 10mm sq cable (45A).

We can do one of two things.

Put a bigger cable in with a 50A circuit breaker – 16mm will carry up to 57A ref method 100. Good luck getting that in the pull switch though. Or…

Use a 45A circuit breaker! This will protect the cable against overload and they are available just not listed in BS7671. They are listed in the On-Site Guide, Table B6 along with the maximum Zs (80%) value.

So, it’s 10 mm sq cable and a 45A Type B BSEN 60898 circuit breaker.

But there’s more. According to Part 7 Section 701 BS7671 (701.411.3.3) we are also going to need Additional Protection using a 30mA RCD.

Other things to consider…

  • Is the supply big enough for the additional load?
  • Will you need to put a separate consumer unit (shower unit) at the distribution board or will it fit into the existing one?
  • Is all of the earthing and bonding satisfactory?
  • Position of the pull switch – at least 600mm away from the edge of the bath or shower cubical if the ceiling is less than 2.25m.

Installing a shower can easily escalate into a ‘big job’ especially if you need to add an additional consumer unit and upgrade all of the earthing and bonding.

Plus don’t forget to inspect and test and it’ll need a Electrical Installation Certificate and also ‘notifying ‘ under Part P as well as it’s a new circuit and also because it’s in the bathroom.

See my other blog posts for max Zs value, types of circuit breakers, additional protection, earthing and bonding etc and also BS7671 for more information if in doubt.

There’s also more information regarding training Part P of the Building Regulations over at our new City & Guilds Part P course website.







Isolation and Switching Chapter 53 BS 7671

Key Points of…

Isolation: 537.2

Every circuit / installation must be provided with a method of isolation from the supply. This isolation device should also be provided with a method to prevent unintentional or inadvertent reinstating (usually lockable).

In a TN-S or TN-C-S system it is not necessary to isolate / switch the neutral conductor providing that it is ‘reliably’ connected to Earth on the distributors incoming supply side.

It’s worth remembering that semiconductor devices can not be used as isolating devices.

Switching off for mechanical maintenance: 537.3

Mechanical maintenance is work that does not involve exposure to electrical connections. As such this work is often undertaken by ‘ordinary persons’.

Examples: Cleaning, adjusting or replacing parts of a machine. Replacing lamps in a fluorescent light.

Isolating devices must be able to switch the full load current and should be local to the equipment. More often than not the main isolating device is also used for mechanical maintenance for machinery as it’s close by and lockable. Other devices may be used such as, fused connection units, double pole switches and plugs and sockets.

Emergency switching: 537.4

Emergency switching should be provided for any part of an installation where it is necessary to isolate the supply quickly to remove unexpected danger.

In installations with rotating machinery this is usually done by using ‘stop’ buttons. Care should be taken to ensure that the stop buttons are…

  • Near to the machine / equipment
  • Should not be capable of being reset remotely, unless with a key switch.
  • If remote resetting is possible then the activated stop button must remain activated at the point of the danger until being reset once the danger is removed.
  • In areas where there are untrained people the stop buttons should lock when operated.

Functional switching: 537.5

The most common type of switching found in an installation. For general use to control any part of an installation that may need to be controlled independently from other parts.

They should be conveniently located and easily operated manually if needed. They include…

  • Light switches
  • Plugs and sockets (below or at 32A)
  • Switched fused spares
  • PIR sensors
  • Time clocks / thermostats etc

Functional switches may not include…

  • Fuses
  • Luminaire connections – plug in type ‘Clix’
  • Unswitched fused spurs
  • Socket outlets above 32A

“These are just the key points. For more detailed information see good old BS 7671 Part 5 Selection and Erection: Chapter 53 Protection, Isolation, Switching, Control and Monitoring”




Question from a student

I have recently signed up for your 17th Edition course and just completed a first fairly quick run through the book with your slides. Will now start again being more thorough and start going through the workbooks. As I have gone through the regs I have “tested” my current home wiring set up (I recently bought the house and it was rewired about 2 years ago) to see how well it complies. This has been a useful aid to my understanding but has raised a few queries in my mind. Specifically:

1. 526.3 is clear about all connections being accessible for inspection.  So would a junction box with screw terminals under floorboards used to connect a spur into a ring power circuit be against the regs? If the junction was an enclosure with say Wago mechanical locking terminals would that then make it more acceptable?

2. I have a ceiling constructed of plasterboard on a galvanised steel U channel structure. Cables simply run over this. The U channel is about 30mm deep and the plasterboard and skim say 15mm so cables don’t meet a requirement of being 50mm above the ceiling 522.6.201. However, pragmatically the screws holding the plasterboard to the framework only penetrate it by about 5mm so no real risk of hitting cable. What would you say is the correct way to run such cabling! How acceptable is pragmatism?

3. 559.5.3.1 through wiring. If a standard ceiling rose with L, N & E in and out and switched line is replaced by say a recessed down lighter which would not typically have all the necessary terminations is it acceptable to make additional connections needed in a suitable enclosure?

Enjoying course,


Hi Rob,

1. It all depends on whether a junction box with screw connections is accessible for inspection and testing or maintenance. If it was just a matter of moving the carpet to one side and popping a floor board up then it could be argued that this is accessible. Adding it’s position to any drawings or on any certification and writing on the floorboard ‘electrical connections below’ could also assist in it being accessible. Or you could as you say use a maintenance free junction box complying with BS 5733 (526.3 vi). Personally, I don’t have a lot of confidence in these things and I would sleep better with a screw terminal junction box under the floor than a MF junction box.

2. This is a common issue where steel frame supports the ceiling. 552.6.201 states that cables should be in a position where they are not damaged by the fixings used to support the ceiling. ie 50mm clearance or one of the methods from 522.6.204 should be used (steel conduit, swa, SELV etc). What we used to do on-site was to run all of the cables through loops made from cable ties attached above the ceiling (quick and cheap) or run cable tray above to maintain a 50 mm clearance.

3. 559.5.3.1 through wiring relates to the practice of wiring a chain of lights through each other. A practice we used to use shop-fitting decades ago where fluorescent lights are mounted in rows butt up to each other with all of the wiring running through the fittings. What you are referring to is the loop in method of wiring a lighting circuit. If you are changing a ceiling rose for a recessed down lighter then you’ll need 559.5.1 (i) to (x) which list all of the recognised methods for connecting lights to the wiring. Have a look at (viii) appropriate terminals enclosed in a box complying with etc…

Hope this helps and I’m glad you’re enjoying the course.

Earth Fault Loop Impedance (Zs)

The earth fault loop impedance (Zs) directly effects the amount of current that flows under earth fault conditions. (Ipf = V / Zs).

As I’ve mentioned in a previous post (see here) fuses and circuit breakers need a certain amount of current in order to meet their maximum disconnection times. ie a 20A type B circuit breaker would require 5 x its rating (100A) in order to meet its disconnection times.

If the earth fault loop impedance is too high then disconnection of the protective device will take too long due to the earth fault current not being high enough for the protective device to meet its maximum disconnection time.

This is why we have the maximum loop impedance (Zs) values in Chapter 41 BS 7671. Remember though. When testing, the results measured are compared to 80% of these value in Chapter 41.

Components of an Earth Loop Impedance Path (Zs)

Zs = Ze + (R1 + R2)

Zs: Overall loop impedance including the source and the installation
Ze: Loop impedance external to the installation
R1: Impedance of the line conductor of the circuit
R2: Impedance of the CPC protecting the circuit



17th Edition Exam Day – Do’s and Don’ts

Just a quick one to clear up a few do’s and don’ts for your exam day…

Do get a good nights sleep and arrive at your centre at least thirty minutes before your exam is due to start.

You’ll need a calculator with a square root function (in case of formula questions) and some photographic ID (passport, driving licence etc). As this is an open book exam you’ll also need to take your copy of BS 7671 (2015 – yellow book).

BS 7671 Regs Book

BS 7671 the 17th Edition Wiring Regulations

Your book may include…

  • Bookmarks (eg post it notes numbered to indicate Chapters or folded corners of pages).
  • Highlighting of text.
  • The 2013 Corrigendum issued by the IET (Although I’ve never seen any exam questions based on it).

Your book can’t contain…

  • Sample exam questions, questions or diagrams.
  • Any writing in the regulations or any accompanying notes.
  • Notes or diagrams or any content that may in any way advantage the candidate when answering questions.

If in doubt consult the centre when you arrive. They may provide you with a clean copy of BS 7671in order to complete your exam.



Periodic Inspection and Testing

In Part 6 Chapter 62 of the 17th Edition BS 7671 we find guidance on completing a periodic inspection and test on an existing electrical installation.

Section 621: 621.1contains the where, why and 621.2 to 612.4 is all of the whats and the who is mentioned in 621.5

Section 622: 622.1 relates to the frequency of periodic inspection and testing, but you’re better off with a copy of Guidance Note 3. 622.2 gives an instance where periodic inspection and testing may not be necessary.

Chapter 63 is all about the Certificates or Reports used

It’s worth remembering that with a Periodic Inspection and Test you are compiling a report on the condition of an existing electrical installation (Electrical Installation Condition Report) 631.2 and are not certifying the compliance anything.

634.1  Along with the Electrical Installation Condition Report you will need to provide a Schedule of Inspection and a Schedule of test results

634.2 Reporting of damage, deterioration, defects, dangerous conditions and non-compliance etc must be recorded. This is done through the use of ‘observation classification codes’…

C1: Danger present. Risk of injury. Immediate remedial action required
C2: Potentially dangerous. Urgent remedial action required.
C3: Improvement recommended
FI: Further Investigation required without delay

The periodic inspection report must contain all of the information contained in the model forms in Appendix 6 of BS 7671: Electrical Installation Condition Report, Condition Report Inspection Schedule and Schedule of Test Results.

See BS7671 and Guidance Note 3 for more details.